Rotating AC electrical equipment connected to the power grid, such as motors and generators, spin in sync or in phase with the grid power. If the rotating equipment becomes out of phase (for example, by a rapid stop-start sequence) or by connecting to the grid without synchronizing the relevant phases, the equipment will be forced almost instantaneously into phase with the grid. This abrupt change is known as an aurora event. The North American Electric Reliability Corporation's (NERC) recognized this vulnerability and issued a Recommendation to Industry in 2011 to the industry to encourage solutions to prevent Aurora events.
Aurora events can result in exceptionally high torques being placed on mechanical components of rotating equipment and associated loads, such as transmissions, pumps, gear boxes, compressors, shafts, turbines, etc. This torque can result in the mechanical components far exceeding their design limitations. Damage or destruction of the rotating equipment and attached devices often results from aurora events. Similarly, electrical windings and components may be damaged by exceptionally high loading during the instant phase realignment event.
High power equipment such as large motors and generators are particularly vulnerable to aurora events because of the high mass rotating components of high power equipment. For example, an 800 kW generator requires substantial mechanical components to transfer mechanical energy and power through the generator and will, therefore, generally have large spinning and moving components that have very high inertial energy when rotating or otherwise moving. When an aurora event occurs, the already rotating equipment attempts to rapidly adjust to a different rotation phase or speed, creating massive torque as the inertial energy of the moving components resists the change to the different phase or speed. Since F=ma, a quick change in speed produces a very high acceleration (a) multiplied by the mass (m) of the moving components creates a very high force (F), which can be exhibited through torque in rotating components. The high force and torque can easily overcome design limitations and destroy critical components of the motor or generator and attached load devices.